  Ground state and finite temperature properties of a system of coupled
frustrated and/or dimerized spin-1/2 chains modeling e.g. the CuGeO$_3$
compound are reviewed. Special emphasis is put on the investigation of the role
of impurity doping. A c hain-mean field computation combining exact
diagonalisations of the chain hamiltonians together with a mean field treatment
of the weak interchain couplings is performed in order to map the microscopic
model onto a low-energy effective model. The latter descr ibes a 2-dimensional
system of effective spin-1/2 local moments interacting by spacially anisotropic
long range spin exchange interactions. An extensive study of this effective
model is performed by Stocastic Series Expansion Quantum Monte Carlo for a wide
range of temperatures and impurity concentrations. Interesting scaling
behaviors of the uniform and staggered spin susceptibilities (above a small
ordering Neel temperature due to a residual 3D coupling) can be interpreted in
terms of the formation of large clusters of correlated spins carrying a finite
magnetization. Such results are reproduced satisfactorily by a new Real Space
RG enabling to deal with long range interactions in two-dimensions
