The Collaborative International Dictionary of English v.0.48:

Pur \Pur\, v. i. [imp. & p. p. Purred; p. pr. & vb. n.
   Purring.] [Of imitative origin; cf. Prov. G. purren.]
   To utter a low, murmuring, continued sound, as a cat does
   when pleased. [Written also purr.]
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "purring":
   beaming, beatific, beatified, birring, blessed, blissful,
   bombilation, bombinating, bombination, booming, burring, buzzing,
   capering, cheerful, chirping, dancing, droning, flushed with joy,
   gay, glad, glowing, happy, hum, humming, joyful, joyous, laughing,
   leaping, low rumbling, radiant, singing, smiling, smirking,
   sparkling, starry-eyed, thrice happy, thrumming, whirring,
   whizzing