Chimico,
In answer to your question, there are a large number of very toxic compounds that are essentially undetectable by their taste or odour.
As you have asked a question that (from our perspective) makes us all a little concerned about your intentions, I have not chosen to give examples of readily available substances that fall into the toxic and tasteless category (of which there are a number), but hopefully the list below will be a guide of some examples that answer your question, but you would not be able to manufacture or use.
Chemical - Nerve agents such as VX, sarin, tabun etc. All without taste (when pure) and extremely toxic.
Biological - Nature provides a whole host of nasty substances, such as ricin, botulinum toxin, PA and LF produced by B. Anthracis and amanitins to name but a few... Lethal doses of many of these are so low that, whether these things taste of anything or not, the levels required to cause harm are well below level detectable by taste.
Radiological - Look up Alexander Litvenenko on Wiki as an interesting example.
Hope this quells your interest.
R