I love using Sequel Pro for MySQL, but also wish it had support for SQLite and PostgreSQL. It's a great app and better than all the other OSX clients that I can find on the internet, including the non-free ones.
It's open source, so nothing's stopping anyone from adding support for other SQL servers.
I have contributed a little bit to Sequel Pro, and I'm a bit familiar with the codebase. Adding support for PostgreSQL is not trivial, a lot of MySQL is hardcoded into the user interface. There have always been many requests for PostgreSQL compatibility, but it would require A LOT of refactoring.
(Disclaimer: I haven't been involved much with SP for almost a year now, so possibly there has been progress in that time)
As one of the current Sequel Pro developers I can backup jabkobob on this one, it's not an easy task. It's not that we're ignoring the requests we get to add support for PostgreSQL or any other database system for that matter, it's simply a matter of time, of which most of the core devs don't have just now. We have a lot of plans around this area (as well as others) for the app, but it's going to take time.
It's open source, so nothing's stopping anyone from adding support for other SQL servers.