The best replacement for google apps is exchange.

The best alternatives to google apps are rated as following:
★★★★★  exchange
★★★★  office 365
★★★★  gmail
★★★  apple apps
★★★  microsoft exchange
★★★  hyperoffice
★★★  sharepoint
★★  outlook
★★  microsoft office
★★  hosted exchange
★  bpos
★  zoho
★  microsoft 365
★  mobileme
★  office
★  exchange 2013
★  google sites
  windows live
  microsoft live
  iphone apps
  yahoo small business
  exchange 2010
  office 365 education
  samsung apps
  exchange server
  microsoft office 365
  google accounts
  microsoft sharepoint
  open office

Your alternative for google apps is not listed? Suggest!
Speaking about google apps, one probably shouldn't forget about outlook.

Other search terms related to google apps and their ratings are:

★★★★★ outlook
★★★★★ godaddy
★★★★ apple mail
★★★ email
★★★ hosting
★★ bluehost
★★ exchange
★★ mx records
custom domain
google voice
mac mail
multiple domains
sales force
microsoft outlook
You have in mind a companion for google apps and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of google apps in all years. Click on graph for more.
google apps
office 365