Numerous Agents of the California Bureau of Investigation (CBI) carry the Glock 26 as their sidearm including Teresa Lisbon (Robin Tunney) and Grace Van Pelt (Amanda Righetti) use these, though they occasionally are seen with Glock 19's.
Glock 19
CBI Agents Wayne Rigsby (Owain Yeoman) and Kimball Cho (Tim Kang) use these as their standard sidearm throughout the first half of season 1 before switching to SIG-Sauer P229's. Agents Teresa Lisbon (Robin Tunney) and Grace Van Pelt (Amanda Righetti) switch between these, and their Glock 26's during season 1. Van Pelt (Amanda Righetti) deciding to stick with her Glock 19 for season 2. Teresa Lisbon (Robin Tunney) keeps one as her backup/off-duty weapon, notably using it to draw out the real killer in the episode "Red Badge".
SIG-Sauer P228
Wayne Rigsby (Owain Yeoman) and Kimball Cho (Tim Kang) switch to these during the second half of season 1.
SIG-Sauer P226
The P226 is carried by a Sheriff's Deputy during a raid in "Red Hair and Silver Tape" (S1E02).
Beretta 92FS
CBI Agent Sam Bosco (Terry Kinney) uses a Beretta 92FS as his primary weapon. Deputy US Marshals draw their Berettas in "Red Sauce" (S1E20).
Smith & Wesson Model 60
A Model 60 is the sidearm of Davis Police Department Detective Steve Preciado (Julian Acosta) in "The Thin Red Line" (S1E08). The gun is said to be chambered in .38 Special.
Smith & Wesson Model 10
The Model 10 is the sidearm of Davis police officer Sam Blakely (Steve Braun) in "The Thin Red Line" (S1E08).
Smith & Wesson Model 36
In "Red Brick and Ivy" (S1E10), a Smith & Wesson Model 36 is used by Patrick Jane (Simon Baker) to force a killer into confessing to the poisoning of Alex Maxwell.
SIG-Sauer P232
A P232 is used by one of the killers during "Red Hair and Silver Tape" (S1E02).
Smith & Wesson SW99
This weapon is carried by the copycat killer impersonating the killer Red John in "Pilot" (S1E01).
Others
Heckler & Koch HK416
A Heckler & Koch HK416 is seen used by the CBI SWAT team during "Red Brick and Ivy" (S1E10), and again in "Red John's Friends" (S1E11).
Remington 870
Seemingly the standard Mentalist shotgun, the Remington 870 is used by Det. Dale Blakely (Mark Rolston) in "The Thin Red Line" (S1E08) and Gina (Frankie Ingrassia) in "Red Sauce" (S1E20), and then Sheriff Hardy (Michael Mosley) and Patrick Jane (Simon Baker) in "Red John's Footsteps" (S1E23), also Cho (Tim Kang) used one during the episode "Red Menace". In each case the shotgun had synthetic furniture, and Sheriff Hardy's had a weapon light in the foregrip.