It supports a long list of databases, a variety of common development languages, has a command history, prompts for variables in statements, and more features I haven't touched. As a Vim plugin, it's fairly light on the CPU, but with all the expressive power you expect from Vim (including syntax formatting/highlighting and the availability of your other favorite plugins). I like it because I can dedicate a plaintext file to a specific task, develop my queries, save my results, and come back to it at a later date if the need arises. Some of these files are an important part of my workflow. The only thing I'd change is a more secure handling of login credentials, but it sure beats the standard command line tools bundled with most databases and requires less overhead than the GUI or web-based ones.
I'm currently working on version 2.0 which includes a table structure editor: https://eggerapps.at/pgcommander-prerelease/
Only in the Ultimate edition though, so quite pricey..
Tora is free, and open source, but a bit rough around the edges.
And to plug my own little endeavour :-) if you're on Linux and are looking for a quick and easy to use Forms builder, here's my early beta http://www.data-forms-action.com/
Supports MySQL and Postgres currently.
Which brings up the question -- most database tools I've used for ad-hoc queries have been mediocre at best. psql isn't bad, but a text based interface is suboptimal in many cases. On the other hand, SquirrelSQL, which I generally use, is adequate though not particularly friendly.
What I'd really love is a UI with some modern sensibilities, like saving all queries in history and providing Sublime Text like cmd-P commands to quickly recall and re-run queries. Auto-limiting, quick scrolling through query history with arrow keys, templated queries/ handling bind params, nice key mappings, SQL formatting, etc...
When I was using Oracle, Toad provided some of the above, but it was pretty bloated and provided way more than what I really needed. Tried navicat and it ate 100% of my CPU and crashed repeatedly. I'd be curious if there are any better (free or cheap) alternatives out there, but most of the offerings I've seen look pretty weak.