Since this thread started, the Internet has made harps in all keys much more easily available, through online outlets such as Amazon.com (which connects you with several other stores for harps they don't keep stocked) and eBay (though the Buy It Now items from most retailers tend to be overpriced). You can sometimes pick up slightly used harps in the less popular keys on eBay, at a good discount.

If you mostly want to play diatonic melodies, there are a few other tunings that fill in some troublesome melodic gaps in the standard (Richter diatonic) tuning, though at the expense of chords (which you might not play anyway). The most popular are the "Paddy" Richter and the Lee Oskar "Melody Maker". The Melody Maker is played in cross-position, providing a different range of available notes relative to the base of the key (the tonic). This means that some tunes will better fit a Paddy and some better fit a Melody Maker, while most tunes are playable on either. Other threads here have discussed (hotly) these other tunings and their relative merits.

As for guitar keys, capoing, harmonica keys and cross-position harps, this could all be easily explained with a few charts, but I'm too lazy at the moment to draw them up. Several charts would be needed, since there are a few basic guitar keys which are generally played without capoing.

The simplest chart is this: the circular set of half-step progressions. Moving the capo up a fret is equivalent to moving up (left) one half-step.

C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/Ab - A - A#/Bb - C - C#/Db - D - D#/Eb - E - F - F#/Gb - G ...

So if you play uncapoed in G, and you put a capo on the 3rd fret, you'd effectively be playing in A#/Bb, and need the corresponding (standard) harp.

Conversely, to find out where you need to capo to play a certain key with more familiar chords, you move right. So to play in Bb, you could capo on the first fret and play in A or capo on the third fret and play in G -- or even capo on the 6th fret and play in E. You'd still need a Bb harp, however.

To play a certain key on harp in cross-position, move 5 half-steps right (up) or 7 half-steps left (down), and that's the harp you'd need. For example, to play A in cross-position, you need a D harp. (Note: Even though Melody Makers are played in cross-position, they are marked according to the key you actually play, so you don't have to bother figuring out the equivalent standard diatonic key.)

BTW, to find the relative minor key, move three half-steps left (down). Conversely, to find the relative major key for a minor key, move three half-steps right (up).

For simplicitly, I'm omitted the enharmonic equivalents of the natural notes (like B# for C), since you rarely encounter them as key or chord names. Basically, a sharp moves you one half-step up (left) and a flat one half-step down (right). Double sharps/flats move you two half-steps.