The kernel automatically loads needed modules and unloads unused ones, so modload(1M), modunload(1M), and modinfo(1M) are not very useful for system administration. However, they can be useful when debugging and stress testing driver load/unload scenarios.
modload(1M) can be used to force a module into memory. The kernel might subsequently unload the module, but modload(1M) can be used to verify that the driver has no unresolved references when loaded. Keep in mind that loading a driver does not mean that the driver will attach. A driver that loads successfully will have its _info(9E) entrypoint called, but will not neccessarily attach.
You can use modinfo(1M) to confirm that your driver is loaded. Here is an example:
$ modinfo Id Loadaddr Size Info Rev Module Name 6 101b6000 732 - 1 obpsym (OBP symbol callbacks) 7 101b65bd 1acd0 226 1 rpcmod (RPC syscall) 7 101b65bd 1acd0 226 1 rpcmod (32-bit RPC syscall) 7 101b65bd 1acd0 1 1 rpcmod (rpc interface str mod) 8 101ce8dd 74600 0 1 ip (IP Streams module) 8 101ce8dd 74600 3 1 ip (IP Streams device) ... $ modinfo | grep mydriver 169 781a8d78 13fb 0 1 mydriver (Test Driver 1.5)
The number in the info field is the major number chosen for the driver. modunload(1M) can be used to unload a module, given a module ID (which can be found in the leftmost column of modinfo(1M) output). A common bug is that a driver refuses to unload, even after a modunload(1M) is issued. Note that a driver will not unload if the system thinks the driver is busy. This occurs when the driver fails detach(9E), either because the driver really is busy, or because the detach entry point is implemented incorrectly.
To remove all currently unused modules from memory, run modunload with a module ID of 0:
# modunload -i 0