If you have Squeezebox server (Logitech Media Server) running (or are willing to run it), whether or not you have any hardware-based Squeezebox devices, grab iPeng as it's the best way to consume/control music from elsewhere on your LAN.

The built-in Video app is barebones and shows a distinct lack of thought in design, but third-party ones aren't much better, except for the fact that some of them will allow you to play containers and codecs the built-in app doesn't support. You won't be doing HD video on that iPad using any of the third-party apps though. I keep 3 or 4 alternative ones and frankly I haven't yet chosen which ones to delete and which one to keep. You may have to do some testing yourself if you want to be able to play alternative formats such as mkv.

I have yet to find an app to consume media from your local network other than iPeng for music as I mentioned - and that requires you to run the Logitech server app. I wish I could recommend something for video and pictures.

For your regular PC/Mac, grab and familiarize yourself with Handbrake - you can get away with using its quick preset to convert things into H.264 in an MP4 container for the iPad. It gets the job done - that's about the best plug I can give it.

It turns out that the iPad is actually a better device for creating content than it is for consuming it, since connectivity options, including apps, are pretty much nonexistent. There's no easy way to move media onto the device other than doing manual syncing as I've outlined above. Moving photos to it from a camera is tedious and borders on completely pointless because of it. If you also have an iPhone and too both up to iCloud, then you can see those photos, a few at a time, via Photostream. But that's also kind of clunky since Apple hasn't really come to grips yet with photos.