who makes a decent center channel speaker?

Not only with the same brand but as similar to the L/R speakers as possible. I prefer and use three identical speakers across the front.

I understand that this might not fit for many people but I advise against the ubiquitous MTM horizontal centers, if at all possible.

Kal

Magnepan makes a nice one ( MMG-C ).

And since you seem to always be on a tight budget, it wont break the bank.

http://www.magnepan.com/

I second Kal - same brand and even better if same type - not always practical though.

"I would stick with the same brand as your left and right channels" (Mlawitm)

"Not only with the same brand but as similar to the L/R speakers as possible. I prefer and use three identical speakers across the front" (Kr4)

"I second Kal - same brand and even better if same type" (Shadorne)

I wouldn't even think twice about using a different center than your mains! You'd be better off either with no center at all, or using some other combo of L/C/R speakers as your multi-channel setup, rather than mixing and matching speakers across your front, yes! (it's basically like haveing a different left and right speaker in a stereo setup - you wouldn't do it).
I basically disagree with the line of thinking of getting a "good center" to go with your mains. This is a SURE way to end up with an incoherent, totally unbelievable soundstage, different sounding pans across the front 3 speakers - drawing you out of the illusion of one soundfield! And yes, every professional audio reviewer will agree here. But, you make your choices, and live with your results, either way.
So what are your left and right speakers, and what's your setup like now?

I agree with the first reply, Mribob. Arial CC5 is one best Center. But if you are asking for a DECENT one, hummm, it'd be lots of them on the menu.... Good luck finding.