Smartphones loaded with 'Guitar Hero' have emerged as critical tools for aspiring rock stars. Now comes the second wave. Call it Guitar Hero: the Real Deal. For a growing number of actual guitarists, their smartphones -- OK, their iPhones -- are suddenly as important as tuners, metronomes or recording equipment, because an iPhone can now do everything these other devices can do, for less money and with less hassle. 'I'm amazed at the quality of some of the apps out there,' said Mike Mueller, contributing editor at Guitar Edge magazine and manager of GuitarInstructor.com. 'At this point it's kind of 'buyer beware,' because everybody's looking to get in on it, but this certainly has legs.' The universe of mobile guitar software can be split roughly into three categories: those that replace traditional guitar accessories like tuners and metronomes; practice apps that simulate a fretboard, and apps that contain chords, scales and tablature. There's even dedicated hardware for attaching your phone to your guitar, but more on that later. With simulators like Guitar: Play and Share ($4), iShred ($5) and Guitarist ($4), players can finger chords, pick strings and hear a fair approximation of how it may sound on the real thing. These can be good for practicing when you're away from your instrument -- that is, if you can get accustomed to them. Jon Francis Lucow, a professional musician in New York City, has used Guitarist and iShred during performances, and to 'record, learn songs and jam on the subway.' He said it took him 'a couple of hours of noodling' to get used to the apps. Given that the display mimics a (cramped) fretboard, you might be tempted to try to hold the device like a guitar, but that would be a waste. The apps are designed more for two-fingered poking than for bar-chord progressions. And because they often include a broad range of sound effects, they're great for building ideas that you can try out when you have the real instrument in hand. Some of the more adventurous musicians, like Lucow, are even plugging their smartphones into amps and performing with them -- although, as he put it, 'it would be hard to make a solo look sexy. And setting your phone on fire means you can't call a cab after the show.' Much more useful, for a less experienced player like me, at least, were apps like GuitarToolkit ($10), TabToolkit ($10), Chordmaster ($2), Scale Wizard ($2) and, to a lesser extent, Jammit ($7). With Chordmaster, users scroll through one of the roughly 7,800 chords in the app's library, and it displays the proper finger placement. Swipe your finger across the screen, and the chord plays. The same is true for Scale Wizard, which includes more than 10,000 scale, mode and arpeggio shapes. Jammit lets you play along with three well-known songs in a given genre, with scrolling tablature that you can set to different speeds. It's nicely designed, but it can be tedious if you're not a big fan of Alice Cooper, Deep Purple or Foreigner, the band's in Jammit's current classic rock version. TabToolkit has similar features, but it allows users to upload digitized sheet music and tablature, so you're not confined to the app's song library. That library, by the way, includes 13 songs, plus eight groups of exercises for warming up and practicing. GuitarToolkit, created by TabToolkit's developer, is among the most popular guitar apps, and rightfully so. It does several things well: the app includes a chord look-up feature that is at least as good as Chordmaster's, but deeper, with more than 500,000 chords. The feature works for eight instruments, including the ukulele and banjo, and users can change the instrument's tuning and find the correct chords for each. The tuner is reliable and easy to use. Perch your iPhone nearby and pluck a string. The display tells you how close you are to the center of a note, and a green light flashes when you're on the mark. The fretboard shows how to play thousands of scales, while the metronome is accurate and flexible enough to offer a variety of sounds and an optional flash. If you don't need all of those functions, you can just buy tuners and metronomes a la carte. They may not come from GuitarToolkit, but fortunately, you can find them from some other highly respected sources. Take Peterson, for one. The company, which produces a popular line of tuners, this year released its first iPhone app, iStroboSoft ($10), which gauges an instrument's accuracy with more precision than most other tuners on the market. Peterson also sells a $13 iPhone cable so you can plug in your instrument, for even greater accuracy in noisy environments. Among the better metronomes is the Visual Metronome ($2), which is the personal favorite of Wolf Marshall, the author of the 'Wolf Marshall Guitar Method' instructional series, and a music lecturer at the University of California, Los Angeles. The app's big, clear display, he said, is especially useful for group performances. When it comes to recording riffs or other compositional snippets, guitarists can rely on something simple, like the device's embedded Voice Memo app. But for more serious compositions, Marshall recommends GigBaby ($1), a four-track recorder with a drum machine. 'You could sketch out some ideas with this,' he said, 'and you've got something you can hand your band.' Most of the apps mentioned are not yet available for Android phones or BlackBerrys, although developers frequently say they are working on adding them. BlackBerry has very few, while Android has a handful of decent guitar-related apps -- mostly tuners and chord libraries -- but nothing yet as good as GuitarToolkit. No matter what device you're using, though, most of the apps pose an inherent problem for musicians: you must either prop your iPhone someplace nearby and squint toward it repeatedly, or just put down your instrument and start pressing buttons. Which is why the Dunlop D65 is a welcome innovation. The device, a swiveling iPhone bracket that suctions to the face of your guitar, is a bit costly ($30 at guitarcenter.com), but the product comes from a company with a long line of guitar-friendly products, and Dunlop says the D65 won't damage common guitar finishes. The bracket also includes a quick-release hinge, just in case your next big finale puts you in the mood to channel your inner Hendrix, yet still have a phone to call a cab with at the end of the show. QUICK CALLS T-Mobile is again selling the Sidekick ($150 with a two-year contract), after a month out of circulation. The phone is popular among social networking fans, but in October a technology glitch led to Sidekick users losing all the data from their devices. The company says Microsoft's Danger unit, which manufactures the Sidekick, has changed its systems to prevent similar problems in the future. ... If typing on a mobile phone is still a challenge for you, test out Verizon's new Samsung Omnia II. It's a touch-screen Windows Mobile phone with a so-called Swype technology, which allows users to type by dragging one finger from each letter to the next, instead pecking at specific letters. (See a demo at bit.ly/IrZgS). Verizon sells the phone for $200, after a $100 mail in rebate and two-year contract. ... Droid owners may now have one less reason for iPhone envy. The Dolphin mobile browser, which is free on the Android Market, has added multitouch capabilities for Droids. Early customer reviews suggest the pinch-and-zoom feature isn't as precise as on the iPhone or the Palm Pre, and some users have complained about unexpected browser shutdowns, so be on the lookout for software updates. 