Navicat is great, it has been surprisingly helpful to have the same database client app for all of the different DB systems (Postgres, MySQL, Sqlite) that I need to interact with.
A bit uglier but if you need to support quite a few databases maybe Razor is another good alternative. I've used it for years. It is not open source though.
Razor is actually quite impressive - I've made it the default SQL client tool for the team.
It works on Windows, OS X and Linux, connects to almost any RDBMS in existence, easily customizable and has tabbed results interface in addition to many other features.
All for $60-70. Easily worth the money, you can get a full trial for 30 days to see if it works for you.