I've been using Mad.Thought for a few weeks, and it just works great. The thing I like is that it provides just the things I need, and does not come with a zillion useless and cumbersome options. Also since it uses a very simple and natural database schema, I was able to pick up entries from the database and display them within my personnal web pages in less than a couple of hours. Very convenient indeed.
An open, cross-platform journaling program.
A scientific plotting package.