Lately I've been using: Essence Studio Nails 24/7 Nail Base
I'm on my second vial of this nail base, that's how good it is. Although I am not a fan of Essence's nail polishes - even though I am currently wearing a Nude from that brand! - I found that the nail base is something completely different. For starters, it sells under 3€, and the quality is quite good.
It is an odorless, colourless base, creamy and easy to apply, it doesn't cake nor runs too thin, and gives a good coverage with just one coat, you do not need a second one. This means it lasts. Good value, in my book. What it does is it preps the nail for the polish. It gives the nail a smoother feel, covering it slightly in a matte coat that seems to grab the polish you layer on top of it, thus making it last longer, and chip less. It also protects the nail, which anyone who tends to wear those low cost brands of nail polish like Kiko, HM, etc., knows is quite essential as these do tendo to wear out the nail surface, making it dry and flaky, frail, breaking easily, and unable to hold on to the colour for long.
So, all in all, it's actually a pretty good run for your money, if you feel like you need to make your nails stronger, and protect them from constant use of nail polish, or even if you want this to last a bit longer in your nails, this is the product to go to if you cannot splash on a high level brand. It does what it sets out to do, and won't bankrupt your finances!