What voidcore posted needs some refinement.
ObjectDock comes in two flavours: free and Plus (paid). The free version is free to get and free to use, but it's not as often and as well looked after as the Plus version.
The Plus version is the one you have to pay for. You can pay a one-off fee and use it forever. You get all the updates in a particular iteration cycle free. For example, when I bought my OD+ it was version just before 1.5 (can't remember which one). I got 1.5 update and 1.9 update free, as well as being able to use the latest beta at no charge either. However, if I wanted to upgrade to 2.0, when it eventually (after over 2 years in development) gets released I'll have to pay. It will most likely be a discounted price since I have already paid for the Plus version. Also, if you buy OD+ a short (no clear timescales here) time before a major iteration (2.0, 3.0 etc) gets released you'll get the upgrade free, but that's at Stardock's discretion.