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.