There is Tunnel and Trolls that use d6's only. Combat is simple character's roll total of weapon damage +combat adds-vs opponent cbt total-highest total wins and the side with the lowest roll take the difference in damage to their characters hit points..I have combined MWWG with tunnel and trolls for my MWWG Campaign I have a 1/2 elf charismatic heroine with a 48 charisma who is very good at giving NOOKIE. To Guys she's interested in doing Nookie with. Combat adds is for every point of Strength-Luck-Dexterity over 10 you add the difference in your attributes together for combat adds total(e.g. character with str 18, luck 17,Dex15 would have a combat total of +20 to any punches or kicks she attacks with including all Melee Weapons. The reason she has a high charisma is elve's in T@T have charisma multipliers of x 2 in charisma. You roll 4d6 for attributes with attributes ranging from 4 to 24 b4 bonuses or penalties are added or subtracted to your character. If I was you I'd get Tunnel and Trolls and use it with Macho Women with Gun's.
Traveller has some solitaire play potential. Trade runs can be solitaire, as can planetoid prospecting. For some of the more contained adventures, it is possible to run solo as well, since interpersonal interactions can be handled by reaction rolls -- i.e. you don't need a referee to tell you if the guild officer likes you or not -- while some adventures are more like dungeon crawls or explorations, with little or no interaction outside of the group.
Traveller Character Generation could very well represent a soul-destroying single-player activity. Tunnels & Trolls, as previously mentioned, positively recommends the prospects of a little solo fun. Given the way Lone Wolf allows you to effectively take experience forward, you could consider that in the same category, too - although Tunnels & Trolls allows for a little more flexibility in that you don't have to play the adventures in any particular order.
Also good and noted for solitaire play was The Fantasy Trip (Wizard & Melee were all that was needed to play most of the solos for it; a company is producing new ones commercially but without license...).
The TFT rules are not commercially available, but are available in PDF from several pirate providers. They are, however, still available inexpensively new and/or used. A variant set of rules is available under a new label from the same commercial adventure provider.
Car Wars had a couple solo adventures: SJG produced Convoy; AutoVentures produced Hell on Wheels.
Traveller is not a solo RPG, but due to heavy rolling possibilities, had lots of solitaire activities. Trade often was played solitaire.
Tunnels and Trolls is still in print; if playing the older adventures, pick up the 5.5 Ed rules from FBI. The 7.0KSA rules are workable, but don't scale the same way, so may result in unbalanced encounters. The 7.0FD rules, included in the same package as the 7.0KSA, are even less compatible with the solos.