The following is a list of notable computer system manufacturers.

ABS Computer Technologies [1] – Gaming systems manufacturer owned by Newegg [2] a popular online parts store.
Acer [3] – Taiwan based third largest computer manufacturer in the world.

[4]

eMachines [5] – Presently a brand name of Acer after being acquired by Gateway [...]

List of information technology companies by market capitalization:
As shown in MacObserver[1]:

Microsoft – $264B[2]
Google – $208B
Cisco – $189B
Apple Inc. – $162B
IBM – $159B
Intel – $155B
HP – $112.57B [3][4]
Dell – $45.09B[5]

References

^ http://www.macobserver.com/article/2007/10/23.12.shtml
^ http://en.wikipedia.org/wiki/List_of_corporations_by_market_capitalization
^ http://finance.yahoo.com/q?s=HPQ&d=t
^ http://www.tuaw.com/2007/07/26/apple-market-cap-passes-hp/
^ http://www.macnn.com/articles/06/01/13/apple.surpasses.dell/