Virii may be one of the methods used in gene therapy, once it finally becomes available. They're an ideal way in inject modified DNA into target cells.
But that's a long way from being everyday usage.

For an everyday use, how about the vaccines?
By injecting a small amount of cowpox virus, people are made immune to smallpox.

Similar thing for many other diseases too.

In computing terms there are - some folks have written a virus to clean up PCs by triggering the download of patches that fix security holes that are currently being exploited on your machine.

Unfortunately, releasing that virus is also an illegal act :-(

Here are a couple of examples. The retrovirus one might have spin offs in human retrovirus protection but I can't see the beneficial aspect of the dwarfing virus in cattle having much of an application in humans unless it could be tailored to control something like gigantism.

http://www.bio.net/bionet/mm/virology/1996-May/006188.html