In Fallout 2, Charisma also determines the number of base companion slots your character is given. This number is equal to your charisma score divided by two, rounded down. For example, if you have 5 Charisma, you can recruit 2 followers. You can still recruit 1 follower at 1 charisma, however.
Predesigned Primary Charisma-based Characters (PPCC) of Fallout are Albert, and in Fallout 2's case, Chitsa.
In Fallout Tactics, Charisma, at best, is a secondary trait. Due to the nature of the game (i.e. almost exclusively combat-based) it is feasible to complete the game easily with 1 or 2 Charisma with no penalty. It is involved with the Barter skill and promotions, and is required for the Leader and Divine Favor perks. A low Charisma, on the other hand, gives access to the Loner perk.
Ways to increase Charisma in Fallout
Find the Singer random encounter. Speaking with Patrick the Celt and having a high enough Speech will increase Charisma by one.
Charisma increases the disposition of all NPCs, which makes speech checks easier. Disposition is also affected somewhat by karma (most NPCs like good characters more), quests (doing a quest in a way an NPC likes can dramatically boost disposition), and dialog options (being a jerk in conversations can lower disposition a little). Charisma also affects the Barter and Speech skills. Characters who use speech will want at least a decent charisma, but adding points to the skill is almost always better than adding points to the score. Characters who do not use the speech skill have little use for this SPECIAL score.
There are also general Charisma checks in dialogue with some characters, in addition to Speech checks. Avoid wearing things like Enclave Power Helmets, as they deduct 1 point of charisma whilst equipped.
Charisma in New Vegas modifies Barter, Speech and Companion Nerve. Companion Nerve gives each companion in the party +5% to their damage and armor, up to a maximum of +50% at 10 Charisma.
Regardless of Barter or Speech level, some interactions will not be available if your Charisma is too low (Ex. Corporal Betsy). These ultimately do not have any significant effect on game play, nor do they provide any special bonuses.