All OpenSim grids are compatible with the Second Life client, which you can download here.
However, it can take a little extra configuration work to use it to access OpenSim grids.
As a result, most OpenSim users currently prefer the Hippo viewer, which is almost identical to the Second Life browser but has the added feature of allowing users to choose the grid they connect to.
You can download the Hippo viewer here.
Two new viewers have recently appeared, both of which allow users to save objects to their hard drives. These are the Meerkat viewer and the Imprudence viewer.
In addition, individual grids may have customized viewers for their users that have been modified to restrict certain types of content, or restrict access to other grids. These viewers are useful in an educational environment, for example, so that students are not able to teleport away to gambling worlds or adult grids.