Some of my colleagues use password databases such as KeyPassX, or the browser's "remember password" feature. These solutions are significantly better than using the same password across all sites, but suffer from a few problems:

• You have all your passwords written down somewhere - at some point you may not be the only one looking at this list
  
• That list isn't always protected - e.g. using Firefox's "remember password" feature without a master password could expose your password on physical theft of the device
• Portability - if you aren't at your machine you can't log in. KeePassX is quite portable, but then ends up making more copies of the password list.
  

None of these are killers, but they aren't ideal. This is where SuperGenPass comes in. SuperGenPass hashes a password with the domain to make a unique password per-site, meaning that a compromise or malicious admin on one site won't give them the password for another site. The main advantage is that there's no database or list of passwords lying around that could be compromised and you only need remember one password. If you wear lots of tinfoil like me, you can remember groups of passwords e.g. critical, important, arb sites. It's ridiculously portable with a bookmarklet (don't use this, see below), data URI, straight JavaScript, Python (alternative) and J2MEE implementations.

There are a few potential problems that need to be considered (they all have solutions) however:

• The bookmarklet runs in the same context as the website you're logging into. This means javascript on the site (or POST'ed/GET'ed information) has access to it. This allows a malicious or hacked site to get hold of your master password. There's a better explanation here, and I put up a demo here. So DO NO USE THE BOOKMARKLET. Any of the other versions are good enough, but are vulnerable to shoulder surfing (something the bookmarklet is not vulnerable to).
• Update: The bookmarklet now uses some random variable and function names to reduce the changes that a straight capture of the master password will work. I'd still avoid the bookmarklet for important stuff.
  
• The algorithm doesn't include za.net and za.org as top level domains. This means that all your passwords for the approximately 30k domains hosted under these could have the same password. As it is unlikely that the majority of internet users use more than one web-app in these domains, this isn't a huge risk. Although, it does technically make finding the correct MD5 collision slightly easier. When I notified the developers of the bug, the response was that a change in the algorithm would affect users with existing passwords on these sites and hence they would not change it. So I made my own, more on that later.
• The default password length is 10. That's fine, but given research like this, and that setting the default to 12 costs the user nothing and potentially buys them $7,700,102,463 extra protection per password, that sounds like a good deal.
• MD5 - this isn't a problem. The knee jerk I hear from some security people is that is uses MD5 and that's broken. However, the vulnerability in MD5 allows other values to be found that hash to the same value (a collision). This does not let you work out the reverse i.e. the original value (master password in this case) from a single hash however. So we're safe here (I think, crypto nerds got any comments?)

In light of the above, I've made my own customised version of sgp. It includes customised versions of all but the J2MEE version (which is partially customised by it's author to include za.net/org already). My recommendation is to use the Data URI as a bookmark loaded in your sidebar (in Firefox). While it is slightly slower logging in to one site because you will need to type the domain, it is faster when logging in to many because you can just change the domain without re-entering your master password. It is vulnerable to shoulder surfing however.

In summary. SuperGenPass provides a convenient way to use different passwords across different sites. There are some potential problems and improvements, to which I recommend using my customised version, with the preferred method being the Data URI as a Bookmark loaded in your sidebar.

Thanks to Russell for pointing SGP out in the first place, and Michael for the Python and J2MEE version and the changes.