Why is o-toluic acid (2-methylbenzoic acid) more acidic than benzoic acid?
Having one methyl group ortho to the carboxylate group will not prevent the carboxylate group from becoming reasonably planar with the aromatic ring.  If you had a second methyl group in the other ortho position, then planarity would be a problem.

A methyl group attached to a benzene ring is electron releasing in both an inductive and a resonance sense. 


Inductively, the electrons in an $\ce{sp^3}$ hybridized methyl group prefer to shift towards the lower energy (more s-character in the orbital) $\ce{sp^2}$ aromatic carbon.  
The methyl group is also electron releasing in a resonance sense due to carbon-hydrogen hyperconjugation (see figure below).  


The resonance structure pictured below on the left is one of many that we can draw for the o-toluic carboxylate anion.  Note that it places a positive charge on the methyl bearing ortho carbon. Both inductively and through resonance (see the hyperconjugated resonance structure below on the right) the methyl group will stabilize this resonance structure.  It is this stabilization provided by the methyl group that makes o-toluic acid a stronger acid (pKa=3.91) than benzoic acid (pKa=4.19).



Note: As pointed out by both @Mithoron and @GaurangTandon, this answer does not explain why p-toluic acid is a weaker acid than o-toluic acid or benzoic acid. The answers in this earlier question better answer this question.