Just a couple of the bigger boys in SA who have spent (and are continuously spending) millions on UNIX systems on a yearly basis (in most cases IBM, which is who I help with implementations), where I have been involved last year:
1. Telkom (quite a couple of P570s, P595s, and recently started to switch to P6s, apart from the stacks of HP SuperDomes and Sun E series)
2. Vodacom (agree with v3g that their computer centers are awesome!)
3. MTN (has an impressive new link between their data centers

)
4. Eskom (Just finished delivering another 4 full P595s there...)
Then smaller, but still substantial:
5. City of Cape Town (was the biggest SAP Billing system worldwide in 2006, and is used as a SAP reference site for this...)
6. Unilever
7. NewClicks
8. Media24
9. Shoprite
...
Thus, lot of sites, lots of money being spent, and lots of money being made!

And all on UNIX!