The Collaborative International Dictionary of English v.0.48:

Ago \A*go"\, a. & adv. [OE. ago, agon, p. p. of agon to go away,
   pass by, AS. [=a]g[=a]n to pass away; [=a]- (cf. Goth. us-,
   Ger. er-, orig. meaning out) + g[=a]n to go. See Go.]
   Past; gone by; since; as, ten years ago; gone long ago.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "ago":
   antiquated, antique, back, back when, backward, blown over, by,
   bygone, bypast, dated, dead, dead and buried, deceased, defunct,
   departed, elapsed, expired, extinct, finished, forgotten, gone,
   gone by, gone glimmering, gone-by, has-been, into the past,
   irrecoverable, lapsed, no more, obsolete, over, passe, passed,
   passed away, past, reminiscently, retroactively, retrospectively,
   run out, since, vanished, wound up