It was rumoured for the longest while and a couple of days ago, it was finally confirmed - Kara Danvers is going to meet Barry Allen.
In the eighteenth episode of Supergirl, titled World's Finest, Grant Gustin's loveable speedster Barry Allen/The Flash will pop up in National City. Now we don't know if this is going to be Barry jumping through different Earth dimensions like he's currently doing in his own show but given that Legends Of Tomorrow recently confirmed the existence of both Batman and Superman in their universe, it's likely that Supergirl also exists within this verse as well (makes sense too).
In less than a year, this is the second crossover with DC characters from different shows and networks. We already had John Constantine rock up in Arrow back in November and he fitted there like a glove and I can imagine that Barry will easily fit into Kara's world and maybe at some point we can get Kara appearing in The Flash too.
Either way, this is very exciting news and kind of show that the DC TV universe is continuing to expand in some rather fun ways.
Press Release: http://www.dccomics.com/blog/2016/02/03/breaking-news-the-flash-is-crossing-over-with-supergirl
US viewers can catch The Flash appearing in Supergirl on March 28th on CBS.