The last issues of QEX have featured two interesting articles by Gary Cobb, G3TMG. He outlines the advantage of using Zolotarev designs for the harmonic suppression filters of transmitters, giving even better suppression of the second harmonic than the more common Chebyshev or quasielliptic filters.

Chebyshev lowpass filter from the GQRP data sheet (issue 1) 
My interest in this was triggered by the test of the Ultimate 3 QRSS/WSPR kit from QRP Labs in the Nov 2016 QST. The review was positive overall, but it was remarked that the
harmonic suppression does not meet FCC requirements (43 dBc or better). I am not sure whether this is due to PCB layout issues, or if better filters can alleviate it, but I note that the design uses the simplest filter of the ones I have listed here.
The evolution of filters for use for harmonic suppression follows at least these three steps:
 Chebyshev type I filters with equiripple in the passband and a monotonically falling, maximally flat stopband. A 7pole version with three inductors and four capacitors in a pinetwork has been around for a while, in e.g. the old recommendation from the GQRP club. It was based on the QST paper "Lowpass filters for amateur radio transmitters," Ed Wetherhold, W3NQN, Dec. 1979. Two designs for a 20 m filter were given there:
1) Max. ripple in passband: 0.00731 dB, attenuation at 28 MHz: 40.7 dB
1) Max. ripple in passband: 0.00960 dB, attenuation at 28 MHz: 34.5 dB

Secondharmonic optimized lowpass filter from the
GQRP data sheet (issue 2) 
An improved stopband was the topic of W3NQN's article "Secondharmonic optimized lowpass filters" in QST Feb. 1999. Here there is one additional capacitor as the central inductor is made into a parallel resonance which gives a zero in the stopband, based on an idea by Jim Tonne, WB6BLD. The design goal is that this zero should be at the second harmonic frequency. W3NQN proposed to call this a Chebyshev filter with a zero (CWAZ) filter, but it is more correct to call it a quasi or pseudoelliptic filter as remarked by G3TMG. It increases the attenuation at the secondharmonic in the 20 m design to better than 60 dB. This design is the basis for the current (Issue 2) GQRP technical sheet. This would also be interesting to test in the QRP Labs Ultimate 3 transmitter kit.

Zolotarev lowpass filter from the Nov/Dec 2016 QEX article 
G3TMG in the new QEX articles has noticed that the passband is overspecified in the above filters as the lower 60% or so of the passband is unused. The Zolotarev design allows for more passband ripple in this part where it does not matter. The advantage is even better stopband attenuation. A measured example for a 60 m filter has a passband ripple of 0.17 0.04 dB, an insertion loss of 0.17 dB, and a stopband attenuation at the second harmonic of 71 dB. This filter has the same component count as the previous one, but the filter is no longer symmetric like the two previous ones. The increase in secondharmonic suppression is not as great as the going from the first filters to the second, but should still be worth the effort.
The papers, which are well worth reading, are:
 Gary Cobb, G3TMG, Zolotarev lowpass filter design, QEX, July/Aug 2016.
 Gary Cobb, G3TMG, A more efficient lowpass filter, QEX, Nov/Dec 2016.