IBM Design
A lot of the IBM choices of fonts came by recommendation of Paul Rand who had a lot of influence on IBM's branding. This article will go through some of the fonts and designs.
Neue Aurora / Aurora Condensed
Most famous for being used as the 'ThinkPad' font, but also used in many numerous other products like the intermediate version of OS/2 pre-WARP and post-Microsoft. Coincidentally Paul Rand designed the multi-coloured OS/2 logo.
Another use was in a handful of PS/2 products like the PS/2E, the PS/2 Note mini-mouse, etc.
Microsoft's marketing team eventually started using this font for early products as well; I'm not sure if that's because the head of design was influenced by Paul Rand, or if they simply looked at what kind of designs IBM was using-- in the same fashion, this is also why Steve Jobs went to Paul Rand for designing the NeXT logo.
Bodoni
Undoubtedly the most commonly used font by IBM. However, IBM uses one variant of Bodoni that has distinct lower case 't' (although IBM's earlier uses of Bodoni used the typical t).