Android - Differences between OsmAnd and OsmAnd+
As of 28/03/2013 the difference is that with the free version is limited to a total of 10 downloads (maps, voice packages, etc) and does not support offline Wikipedia content.
I know because I tried the free version and then buyed the plus one to be able to download entire european countries. Also I think is a really good application for 7 euros.
As of today (24/03/2016), OsmAnd has 3 versions publicly available. This is possible because OsmAnd is GPL software, and therefore can be freely compiled and distributed.
1: Developers' free version, crippled to 10 downloads and no Wikipedia capability.
2: Developers' paid version, uncrippled.
3: F-Droid version, uncrippled but has slower updates than the paid version.
There exist 3 Versions of Osmand
The Market free Version: full featured
The Market Plus version: full featured, used as a way to donate to the developers as incentive to keep on improving.
The F-Droid Version: A source compiled free software version by fdroid with almost no changes to the normal version. Not so frequent app updates. The maps are updated the same way as in the offical build.
Do not under any circumstances install two versions at the same time, it will seriously destroy both of the apps, the map files on sd, your sanity and your battery power.