I recently bought one of the specialist pocket hole clamps for that purpose, as one specific thing it allows is clamping a T-joint, such as the middle shelves of a bookcase.
Obviously pointless if you're not doing pocket-hole joinery, but dead handy if you are (that said, haven't used it yet!). And better on an assembly table, rather than just clamping a workpiece in 'free space'.
Just in case you're tempted, I've got a couple of these strap clamps with corner pieces:
The weak point is on the corner piece with the handle: where the straps are turned to run along the sides of the work, the thin plastic mouldings take the force of the entire clamp. Stanley of old would have been utterly ashamed of such a weak and stupid design. Mine snapped away at the sides (predictably) and they now have bolts through instead (waiting for those to pull out of the plastic sometime soon, too). And the other corners don't let the strap slide nicely to keep an even tension. Total waste of time and money.
The Veritas one looks good (and pretty good value), but you'll still need something on a diagonal, probably, to pull and keep everything true. And you also need to pull things like picture frames down to a reference surface, so everything glues up in the same plane.
I really like the ones in that Youtube video Marcros posted, too - dead easy to make and use. OK, you can't do more than one joint at a time, but it's arguable that, for awkward frames, it's easier to do two halves carefully (then mate them), than try to get everything bang-on in one big scramble. Again it all depends on the size of the frame, and what it's for.
Just my twopence.
E.
PS: The Bessey one looks far better than the Stanley, but... it will still suffer from one overall design issue: the force will be reduced by the number of corners, and the angles involved (if you're using the corner pieces). On a per-corner basis it won't actually be applying a fraction of the force you could get with a screw clamp and 45deg wedges (like the ones in the YouTube video), and it will still try to pull everything out of square too, because of the friction at the corner pieces, like my Stanley ones do.
