For me, every spring, I do three main things.
1. Declutter. I go thorough drawers, closets, shelves, and anywhere else and decide if we still need something, or if it can be donated/sold on craigslist/whatever. We have a small house and accumulate too much stuff. Also, I think it is much much easier to keep a house clean when you don't have too much stuff.
2. I move everything. I get underneath furniture I haven't cleaned under and behind.
3. Deep cleaning that doesn't need to be done all the time. Things like oven cleaning (which is hard to do in an eco-friendly manner and takes a huge amount of elbow grease. I'd love to know what other people do as far as that goes), cleaning light fixtures, windows (including outside), etc.
I'm always surprised when the sun really comes out what I see that I didn't see during the winter. I find dirty marks on walls and other dirt that I hadn't seen before.
The good thing about having a small house is that it doesn't take huge amounts of time to do this kind of work.
What do you do as far as spring cleaning goes?