user4334: Is there an online simulator that would allow me to step through, debug, or simulate the execution of a Bitcoin script?

user26327: There is a "Script Playground":
http://www.crmarsh.com/script-playground/

user56438: The github repository for the first link posted by StephenM347 can be found here
If you want to run it locally, clone the repo and then,

cd into /path/to/bitcoinIDE/build
Launch a server from here. This will launch a server on port 8000. 
python -m SimpleHTTPServer 8000

From the browser, visit localhost:8000, and select the file editor.html


user92061: I have a tool at http://www.bowmain.co.uk/BSV/ which may be use. Let me know what you think as I can make changes to it.

user18196: I know of these two:

https://webbtc.com/script
http://www.cs.princeton.edu/~tongbinw/bitcoinIDE/build/editor.html
https://siminchen.github.io/bitcoinIDE/build/editor.html
https://bitauth.com/ide/

The first one in particular I have found to be very useful.

user7414: have a look at BitcoinIDE (on github.io, working as of Apr 2018)
p,s:
as @Samarth Hattangady mentioned, the source code can be found at this repo if you want to run it locally.

user5841: I've been looking for a tool to experiment with bitcoin script for a while now, and I finally finished developing one myself.
BitAuth IDE is an open source project for designing advanced scripts for bitcoin and bitcoin-like cryptocurrencies. It’s both a learning sandbox and a tool for designing new kinds of wallets. 
It includes a live-updating, interactive, line-by-line debugger, which visualizes that state of the stack over the entire execution of the program. There's also an entity and variable system for automatically generating private keys, public keys, and correct signatures across locking and unlocking scripts. 
See here for a more detailed tour and a video walkthrough.
You can use the online demo of BitAuth IDE.
And the source is on GitHub.
I've also assembled a much longer Bitcoin Script Reading List, which you might find useful. It includes links to several more related tools and script testing environments.

user60443: Highly recommend btcdeb by kallewoof. Among other things, you can step through scripts including verify signatures. It is written in c++ and compiles using autotools.
btcdeb
Bitcoin Script debugging utilities.
This is a set of tools used to debug or construct scripts for use in Bitcoin.

btcdeb> help
step     Execute one instruction and iterate in the script.
rewind   Go back in time one instruction.
stack    Print stack content.
altstack Print altstack content.
vfexec   Print vfexec content.
exec     Execute command.
tf       Transform a value using a given function.
print    Print script.
help     Show help information.


4334: <p>Is there an online simulator that would allow me to step through, debug, or simulate the execution of a Bitcoin script?</p>
 26327: <p>There is a "Script Playground":
<a href="http://www.crmarsh.com/script-playground/" rel="nofollow">http://www.crmarsh.com/script-playground/</a></p>
 56438: <p>The github repository for the first link posted by StephenM347 can be found <a href="https://github.com/siminchen/bitcoinIDE" rel="nofollow noreferrer">here</a></p>

<p>If you want to run it locally, clone the repo and then,</p>

<ol>
<li><code>cd</code> into <code>/path/to/bitcoinIDE/build</code></li>
<li><p>Launch a server from here. This will launch a server on port 8000. </p>

<pre><code>python -m SimpleHTTPServer 8000
</code></pre></li>
<li><p>From the browser, visit localhost:8000, and select the file <code>editor.html</code></p></li>
</ol>
 92061: <p>I have a tool at <a href="http://www.bowmain.co.uk/BSV/" rel="nofollow noreferrer">http://www.bowmain.co.uk/BSV/</a> which may be use. Let me know what you think as I can make changes to it.</p>
 18196: <p>I know of these two:</p>
<ul>
<li><strike>https://webbtc.com/script</strike></li>
<li><strike>http://www.cs.princeton.edu/~tongbinw/bitcoinIDE/build/editor.html</strike></li>
<li><a href="https://siminchen.github.io/bitcoinIDE/build/editor.html" rel="nofollow noreferrer">https://siminchen.github.io/bitcoinIDE/build/editor.html</a></li>
<li><a href="https://bitauth.com/ide/" rel="nofollow noreferrer">https://bitauth.com/ide/</a></li>
</ul>
<p>The first one in particular I have found to be very useful.</p>
 7414: <p>have a look at <a href="https://siminchen.github.io/bitcoinIDE/build/editor.html" rel="nofollow noreferrer">BitcoinIDE</a> (on github.io, working as of Apr 2018)</p>

<p>p,s:</p>

<p>as @Samarth Hattangady mentioned, the source code can be found at <a href="https://github.com/siminchen/bitcoinIDE" rel="nofollow noreferrer">this</a> repo if you want to run it locally.</p>
 5841: <p>I've been looking for a tool to experiment with bitcoin script for a while now, and I finally finished developing one myself.</p>

<p>BitAuth IDE is an open source project for designing advanced scripts for bitcoin and bitcoin-like cryptocurrencies. It’s both a learning sandbox and a tool for designing new kinds of wallets. </p>

<p>It includes a live-updating, interactive, line-by-line debugger, which visualizes that state of the stack over the entire execution of the program. There's also an entity and variable system for automatically generating private keys, public keys, and correct signatures across locking and unlocking scripts. </p>

<p>See <a href="https://blog.bitjson.com/bitauth-ide-write-and-debug-custom-bitcoin-scripts-aad51f6e3f44" rel="nofollow noreferrer">here for a more detailed tour and a video walkthrough</a>.</p>

<p><strong>You can use the <a href="https://bitauth.com/ide/" rel="nofollow noreferrer">online demo of BitAuth IDE</a>.</strong></p>

<p>And the <a href="https://github.com/bitjson/bitauth-ide" rel="nofollow noreferrer">source is on GitHub</a>.</p>

<p>I've also assembled a much longer <a href="https://blog.bitjson.com/bitcoin-script-a-reading-list-656242edfa6c" rel="nofollow noreferrer">Bitcoin Script Reading List</a>, which you might find useful. It includes links to several more related tools and script testing environments.</p>
 60443: <p>Highly recommend <a href="https://github.com/kallewoof/btcdeb" rel="noreferrer">btcdeb</a> by kallewoof. Among other things, you can step through scripts including <a href="https://github.com/kallewoof/btcdeb#signature-checking" rel="noreferrer">verify signatures</a>. It is written in c++ and compiles using autotools.</p>

<h2>btcdeb</h2>

<p>Bitcoin Script debugging utilities.</p>

<p>This is a set of tools used to debug or construct scripts for use in Bitcoin.</p>

<p><a href="https://i.stack.imgur.com/Qm41h.gif" rel="noreferrer"><img src="https://i.stack.imgur.com/Qm41h.gif" alt="enter image description here"></a></p>

<pre><code>btcdeb&gt; help
step     Execute one instruction and iterate in the script.
rewind   Go back in time one instruction.
stack    Print stack content.
altstack Print altstack content.
vfexec   Print vfexec content.
exec     Execute command.
tf       Transform a value using a given function.
print    Print script.
help     Show help information.
</code></pre>
