There are many examples, but these came to my mind right now.
1) Twitter:
Twitter has been growing at a spectacular pace in 2010 in Japan. They now have well over 10 million users, and I wouldn’t be surprised if the real number would be closer to 15 million (Twitter isn’t providing specific numbers).
Here’s some background (written by me) why: http://accjjournal.com/micro-blo…
2) Mobile social gaming:
Definitely one of the hot areas in Japan’s web last year. The two biggest companies in this area, DeNA (with Mobage-town) and GREE are running from one record (in terms of revenue and profit) to the next.
But there are also many startups producing social games now (especially for mobile phones), for example Pikkle, Istpika, Pokelabo, Gumi, Geisha Entertainment, etc. etc.
3) Daily coupon services:
The first Groupon clone in Japan, Piku, started in April and since then, Japan has been flooded with similar clones. According to one of the owners of a leading service in this area, there are now 160 Groupon clones operating in Japan.
Groupon Japan itself is said to have generated a whopping $12 million in sales in December.
4) Evernote
See Kyohei Suzuki’s answer. One reason for the popularity is that Evernote’s CEO actually takes Japan as a market seriously, for example by visiting the country regularly and establishing an office over here.
5) Togetter
Developed single-handedly by a person called Toshiaki Yoshida, this Twitter-based curation service went from zero (in September 2009) to 2 million unique visitors monthly by December 2010.
The English version is called Chirpstory: http://chirpstory.com/
6) Ameba Pigg
This is a virtual world developed by Tokyo-based Internet giant CyberAgent. In October 2009, there were 1.4 million members. By June 2010, this number ballooned to over 4 million (no newer stats available).
7) Other trends
Smartphones (iPhone, Android) and the iPad have become more popular in 2010, along with apps and services tailor-made for these devices.
Digital publishing has become more popular last year (e-books and magazines, digitization of existing printed content etc.).
Augmented reality is another trend that picked up speed last year in Japan, led by Tonchidot’s Sekai Camera aplication for the iPhone and Android (quite a few other startups and larger companies have followed up with AR apps after them).
