I'm a standing desk guy, but a lot of what I've read suggests that the best possible situation at a desk is combination of both sitting and standing. Without an adjustable desk, this isn't exactly easy to accomplish, and even if you do, it becomes far too easy to stay in the sitting position much longer than you were standing.
Focal Upright, a company known for making unique ergonomic furniture, has released their latest stool, the Mobis. This is a flexible, height adjustable "leaning stool" meant for people that stand all day but need a break from being on their feet once in a while. Leaning on it keeps your major muscles extended, which reduces lower back strain and because it's flexible, it helps keep your core engaged even while you're at rest.