user48765: I realize that this question may be borderline bannable because it's asking for suggestions on tools, but it will really help newbies. This online tool allowed me to play around with hashes and to really understand them at a deeper level: http://www.fileformat.info/tool/hash.htm
I can't find a similar tool (that works) for ECDSA cryptography where I can play around with public and private keys, and do digital signatures on messages, and test signature verification.
I've found these 2 sites that claim to do this but didn't work for me:

http://dbis.rwth-aachen.de/~renzel/mobsos/lib/js/jsrsasign/sample-ecdsa.html
http://extranet.cryptomathic.com/ecc/index

Thoughts? Suggestions?

UPDATE: Great suggestions below. Some tools I found:
for hashing:
http://www.fileformat.info/tool/hash.htm
generating public private key pairs and decrypting ciphers:
https://8gwifi.org/rsafunctions.jsp
Generating EC public private key pairs and signing Verifying the Message 
https://8gwifi.org/ecsignverify.jsp
generating public private key pairs and testing signing:
https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html

user100710: Here is an ECC calculator for the Secp256k1 curve that I'm programming in Java.
https://github.com/MrMaxweII/Secp256k1-Calculator

user38482: closest I can come up with are these:
http://coinig.com/
http://gobittest.appspot.com/Address 
https://www.bitaddress.org/bitaddress.org-v3.2.0-SHA256-ad4fd171c647772aa76d0ce828731b01ca586596275d43a94008766b758e8736.html 
https://coinb.in/
not exactly ECDSA, but served me alot for Bitcoin undertsanding/testing.

Security hint: you would never provide/use a private key to/from such a public website, unless you are really sure of what you are
  doing. Be extremly careful.


user1232: Here are a few I'd recommend:

https://bitcore.io/playground/#/address (private key - public key)
https://brainwalletx.github.io/#generator (signing and verifying but bitcoin specific)

Or play with pybitcointools at the command line (not online):
$ git clone https://github.com/vbuterin/pybitcointools.git
$ cd pybitcointools
$ python
>>> from bitcoin import *
>>> sk = random_key()  # Generate a private key
>>> vk = privtopub(sk) # Generate a public key
>>> msg = 'hello world' # Create a simple message
>>> sig = ecdsa_sign(msg, sk) # Sign the message using your private key
>>> print sig
GxXGAt...2L/eJk=
>>> print ecdsa_verify(msg, sig, vk) # Use sig and public key to verify
True
>>> msg = 'hello mars' # Change the message
>>> print ecdsa_verify(msg, sig, vk) # Changing the msg invalidates sig
False

http://showterm.io/203b168061b0156c4d1dd
HD Wallet stuff:

https://iancoleman.github.io/bip39/
http://bip32.org/

If you're looking to learn more about ECDSA suggest checking out: https://jeremykun.com/2014/02/08/introducing-elliptic-curves/

48765: <p>I realize that this question may be borderline bannable because it's asking for suggestions on tools, but it will really help newbies. This online tool allowed me to play around with hashes and to really understand them at a deeper level: <a href="http://www.fileformat.info/tool/hash.htm" rel="noreferrer">http://www.fileformat.info/tool/hash.htm</a></p>

<p>I can't find a similar tool (that works) for ECDSA cryptography where I can play around with public and private keys, and do digital signatures on messages, and test signature verification.</p>

<p>I've found these 2 sites that claim to do this but didn't work for me:</p>

<ol>
<li><p><a href="http://dbis.rwth-aachen.de/~renzel/mobsos/lib/js/jsrsasign/sample-ecdsa.html" rel="noreferrer">http://dbis.rwth-aachen.de/~renzel/mobsos/lib/js/jsrsasign/sample-ecdsa.html</a></p></li>
<li><p><a href="http://extranet.cryptomathic.com/ecc/index" rel="noreferrer">http://extranet.cryptomathic.com/ecc/index</a></p></li>
</ol>

<p>Thoughts? Suggestions?</p>

<hr>

<p>UPDATE: Great suggestions below. Some tools I found:</p>

<p>for hashing:
<a href="http://www.fileformat.info/tool/hash.htm" rel="noreferrer">http://www.fileformat.info/tool/hash.htm</a></p>

<p>generating public private key pairs and decrypting ciphers:
<a href="https://8gwifi.org/rsafunctions.jsp" rel="noreferrer">https://8gwifi.org/rsafunctions.jsp</a></p>

<p>Generating EC public private key pairs and signing Verifying the Message 
<a href="https://8gwifi.org/ecsignverify.jsp" rel="noreferrer">https://8gwifi.org/ecsignverify.jsp</a></p>

<p>generating public private key pairs and testing signing:
<a href="https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html" rel="noreferrer">https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html</a></p>
 100710: <p>Here is an ECC calculator for the Secp256k1 curve that I'm programming in Java.
<a href="https://github.com/MrMaxweII/Secp256k1-Calculator" rel="nofollow noreferrer">https://github.com/MrMaxweII/Secp256k1-Calculator</a></p>
 38482: <p>closest I can come up with are these:</p>

<p><a href="http://coinig.com/" rel="nofollow noreferrer">http://coinig.com/</a></p>

<p><a href="http://gobittest.appspot.com/Address" rel="nofollow noreferrer">http://gobittest.appspot.com/Address</a> </p>

<p><a href="https://www.bitaddress.org/bitaddress.org-v3.2.0-SHA256-ad4fd171c647772aa76d0ce828731b01ca586596275d43a94008766b758e8736.html" rel="nofollow noreferrer">https://www.bitaddress.org/bitaddress.org-v3.2.0-SHA256-ad4fd171c647772aa76d0ce828731b01ca586596275d43a94008766b758e8736.html</a> </p>

<p><a href="https://coinb.in/" rel="nofollow noreferrer">https://coinb.in/</a></p>

<p>not exactly ECDSA, but served me alot for Bitcoin undertsanding/testing.</p>

<blockquote>
  <p><strong>Security hint</strong>: you would never provide/use a private key to/from such a public website, unless you are really sure of what you are
  doing. Be extremly careful.</p>
</blockquote>
 1232: <p>Here are a few I'd recommend:</p>

<ul>
<li><a href="https://bitcore.io/playground/#/address" rel="nofollow noreferrer">https://bitcore.io/playground/#/address</a> (private key - public key)</li>
<li><a href="https://brainwalletx.github.io/#generator" rel="nofollow noreferrer">https://brainwalletx.github.io/#generator</a> (signing and verifying but bitcoin specific)</li>
</ul>

<p>Or play with <code>pybitcointools</code> at the command line (not online):</p>

<pre><code>$ git clone https://github.com/vbuterin/pybitcointools.git
$ cd pybitcointools
$ python
&gt;&gt;&gt; from bitcoin import *
&gt;&gt;&gt; sk = random_key()  # Generate a private key
&gt;&gt;&gt; vk = privtopub(sk) # Generate a public key
&gt;&gt;&gt; msg = 'hello world' # Create a simple message
&gt;&gt;&gt; sig = ecdsa_sign(msg, sk) # Sign the message using your private key
&gt;&gt;&gt; print sig
GxXGAt...2L/eJk=
&gt;&gt;&gt; print ecdsa_verify(msg, sig, vk) # Use sig and public key to verify
True
&gt;&gt;&gt; msg = 'hello mars' # Change the message
&gt;&gt;&gt; print ecdsa_verify(msg, sig, vk) # Changing the msg invalidates sig
False
</code></pre>

<p><a href="http://showterm.io/203b168061b0156c4d1dd" rel="nofollow noreferrer">http://showterm.io/203b168061b0156c4d1dd</a></p>

<p>HD Wallet stuff:</p>

<ul>
<li><a href="https://iancoleman.github.io/bip39/" rel="nofollow noreferrer">https://iancoleman.github.io/bip39/</a></li>
<li><a href="http://bip32.org/" rel="nofollow noreferrer">http://bip32.org/</a></li>
</ul>

<p>If you're looking to learn more about ECDSA suggest checking out: <a href="https://jeremykun.com/2014/02/08/introducing-elliptic-curves/" rel="nofollow noreferrer">https://jeremykun.com/2014/02/08/introducing-elliptic-curves/</a></p>
