Google Drive - mobile, cloud-based method of always having copies of your passport, visas, ID, insurance, and even social security card on you at all times.
Mobile translator - look into the most recommended brands for your respective travel
Twitter/Instagram - blogging is passe; "micro-blogging" services make it easier to share your experience and to connect with friends with limited internet access, less time to interface with social media, etc.
Aviary - use this app to show off your trip through stunning filters.
AirBnB - if you're looking to splurge and find an apartment for yourself or a small group of friends for a short time, this is the chicest service provider in that arena *download their mobile, social app