Bloodclaw Master (Book of Nine Swords) grants Shifting 1/day at 1st-level for +2 Strength and such, temporarily, with additional uses per day as they advance in the PrC. War Mind (Expanded Psionics Handbook and the SRD) gets temporary Strength and Constitution boosts starting at 1st-level in the PrC.
Racial paragon classes (3-level classes detailed in Unearthed Arcana and the SRD) give +2 to a certain ability score at 3rd-level.
Tattooed Monks (Oriental Adventures, possibly reprinted in 3.5 CW or something?) can gain temporary ability score boosts from certain tattoos. Singh Ragers (OA) get a rage-like ability called Lion's Fury that boosts Str/Con. Shintao Monks (OA) can boost one of their ability scores for a few hours once per day, starting at 1st-level. A few other OA prestige classes get Rage or similar abilities that boost Strength and such.
The Fist of Hextor (Sword & Fist, possibly reprinted in 3.5 CW or CD?) gets a +4 Strength bonus a few times per day, for several rounds each time, starting at 2nd-level. The Pale Master (Tome & Blood) gets +4 inheren Strength at 4th or 5th level from an undead graft.
Probably some others in 3.0 and 3.5 books that I just don't have.