The Android Market now features a web store, where you will be able to freely browse the wondrous collection of free and paid apps.
Should you decide that a particular app is worthy of your hard-earned - go ahead and purchase, and the app will be remotely installed to your phone/tablet over the air (OTA).
Perhaps, however you want to try out the free version first. But "Oh what a pain" you cry, realising you will have to then un-install the free and download the full. No more! Google has freed you of such hindering tasks; the in-app purchases SDK was released today, allowing developers to include support for buying full versions, or add-in packs from within the app itself.
No more themes littered throughout the Market!