Four sources for audio and video streams

Here you can have access to the NPR of the NYC region.  It could be useful if you want to improve your listening comprehension of American English.  You can listen to the live FM broadcast or go to their show archives.  I personally like Radiolab, Selected Shorts and Fresh Air among the shows they air.
Here you can have access to the BBC website, which also airs on the WNYC in the mornings Brussels time.  It could be useful to improve your listening comprehension of British English.  They also have links under "Learning English", where you can find numerous tools for learners of English as a second language.
Here you can have access to the audio and video streams on The Economist website.  This again is a good source for improving British English listening comprehension.
Here you can have access to TED which is a good source for numerous talks on a very wide range of topics.  Most talks are given by Americans and Canadians, but there are also other speakers of different nationalities, including some British.

Conversation Exchange

Here you can have access to the Conversation Exchange website.

What and which

Here you can find a document with information on the use of 'what' and 'which'.

Modal verbs

Here you can find some information and exercises on modal verbs along with the key for the exercises. (Source: English Grammar in Use. Intermediate by Raymond Murphy)

Here you can find more exercises on modal verbs.

Relative, -ed and -ing clauses

Here you can find some exercises on relative, -ed and -ing clauses along with the key.

Gerund vs. infinitive

Numerous exercises on gerund vs. infinitive along with their key are hereby made available for you to have access at your convenience.  You can also find the list of verbs used with gerund and/or infinitive here.

Here you can find more information and exercises on the same topic.

Telephone conversations

Here you can have access to the BBC webpage on telephone conversations.
Here you can find more material on the same topic.